Annular extrusion die with internal choke ring and spider mandrel

Abstract

An annular extrusion die for extruding tubular thermoplastic materials, such as foamed thermoplastic, which incorporates an internal spider mandrel and a novel choke ring for eliminating striations in the extruded thermoplastic material while imparting an improved control over the uniformity in the gauge or thickness of the extruded plastic material. In order to more accurately control the uniformity in the gauge of the extruded plastic material, rather than varying the width of the gap or restriction in the die flow passageway formed by the choke ring which, in response to even minor changes in the gap width considerably changes the flow quantity, may overcompensate or overcontrol variations in the gauge thickness; there is varied the length of the land of the choke ring in the direction of material flow, in effect, in the machine direction, downstream of the spiders supporting the internal mandrel.

What is claimed is: 
     
       1. An annular extrusion die for the extrusion of a thermoplastic material, such as foamed plastic and the like, comprising a die body having an elongated cylindrical cavity having one end communicating with a source of thermoplastic melt and with an extrusion die orifice for extruding tubular product at the opposite end thereof; an elongated cylindrical mandrel centrally arranged within said cavity and defining and elongate annular flow passageway therebetween; a plurality of circumferentially spaced said spider means extending radially across said flow passageway for fastening said mandrel to said die body; and a choke ring interposed in said annular flow passageway downstream of said spider means to form an annular restriction in the cross-section of said flow passageway and producing a back pressure on the thermoplastic material upstream of said restriction, said choke ring having a land extending in the flow direction of said passageway, said land having a height extending parallel to said flow direction of said passageway and said land continually varying in height about the circumference thereof so as to impart a predetermined varying back pressure distribution along the circumference of said restriction to the thermoplastic material conveyed through said restriction. 
     
     
       2. An annular extrusion die as claimed in claim 1, wherein the height of the land of said choke ring changes in a sinusoidal configuration about the circumference of said restriction. 
     
     
       3. An annular extrusion die as claimed in claim 2, wherein the height of the land about the circumference of said restriction changes in accordance with the equation   L=A-[B (cos 2 θ)]     in which   L=[length] height of the land parallel to said flow direction at a specific point about the circumference of said annular restriction relative to the circumferential location of the spider means,   A=nominal basic height of the land,   B=a constant proportional to A, calculated such that 0.05<B/A<0.20.   
     
     
       4. An annular extrusion die as claimed in claim 3, wherein the greatest height of the land of said choke ring is circumferentially located intermediate the circumferential location of each of said spiders, and the minimum height of said land is in alignment with each of said spider means. 
     
     
       5. An annular extrusion die as claimed in claim 1, wherein each of said spider means has an aerodynamic cross-section in the direction of material flow through said annular flow passageway. 
     
     
       6. An annular extrusion die as claimed in claim 1, wherein said cosine choke ring is mounted on said die body and includes an annular protuberance extending radially inwardly into said flow passageway to form said annular restriction. 
     
     
       7. An annular extrusion die as claimed in claim 6, wherein said land is formed on said annular protuberance, said protuberance being configured to impart said varying height to said land in the flow direction of said thermoplastic material. 
     
     
       8. An annular extrusion die as claimed in claim 1, wherein said cosine choke ring is mounted on said internal mandrel and includes an annular protuberance extending radially outwardly into said flow passageway to form said annular restriction.